Carrier current telephone apparatus



Dec, 36, 1947. R. K. CROOKS Q 2,432,560

CARRIER CURRENT TELEPHONE APPARATUS Filed March 18, 1946 INVENTOR.Haiphii poahw.

' ari iwg Hi5 Ammwx Patented Dec. 16.1947

UNITED STATES PATENT OFFICE CARRIER CURRENT TELEPHONE APPARATUS Ralph K.Crooks, OHara Township, Allegheny Cunty,.Pa., assignor to The UnionSwitch & Signal Company, Swissvale, Pa., a corporation of PennsylvaniaApplication March 18, 1946, Serial No. 655,071 13 Claims. (01. 1792.5)

1 2 My invention relates to communication appathe transmitting tube toprovide a substantially ratus, and more particularly, to portablecarrier uniform degree of modulation for all settings. current telephoneapparatus. Other objects, features and purposes of my in- The principalobject of my invention is the vention will be pointed out as thedescription proprovision of apparatus suitable for use on rail- 5 ceeds.

roads or elsewhere for emergency communica- I shall describe two formsof apparatus emtion between two local stations or between such bodyingmy invention, and shall then point out stations and a central ofiiceover existing telethe novel features thereof in claims. phone, telegraphor signal line circuits without Referring to the accompanyin drawings,Fig. interference with the services normally using 1 illustrates oneform of apparatus embodying such circuits, or under conditions involvingthe my invention in which the switching from the failure of the usualcommunication circuits. receiving to the transmitting condition isaccom- In its preferred form my invention is embodied plished, in part,by the provision of a relay R in in a set of electron tubes andassociated appathe unit. while Fig. 2 illustrates another form in ratusmounted in a readily portable unit to which which the switching is o ped o y by a telephone hand set is attached. Power may changing thepotentials appli d t the t e be supplied to the unit by connecting it toan al- Similar reference characters refer to similar ternating currentlighting circuit, and the unit parts in both Viewshas two line terminalsby which it may be con- Referring to Fig. 1, the reference character Inected between a pair of wires on a pole line, or designates a twin tubecontaining in a single bulb between a line wire and ground, to providevoice two voltage amplifier triodes VI and V2, while communicationbetween the set and one or more tube V3 is a power amplifier tetrode.Direct cursimilarly connected units at distant points to rent issupplied to the positive terminal B of the which the line wires extend.unit, and to the negative or ground terminal G, The units are adapted totransmit and receive connected to the frame of the unit, by a poweramplitude modulated carrier currents of a suitsupply it of v n l mcomprising a able frequency such as 35 kilocycles per second.transformer h g its p i y Winding 2 All units employ the same carrierfrequency, and ranged for connection to the local alternating each handset is provided with push-to-talk current lighting circuit as indicatedby the legend button to change the unit fr m th receiving t A. C. on itsterminals. The end terminals of the transmitting condition. No callingdevice'is e o d y Winding 3 are connected to t e needed, as theapparatus is intended principally plates of a f Wave rectifier t d thefor use under conditions where an operator will middle terminal ofsecondary winding 3 is conbe listening for calls. nected to ground. Thefilament of tube V4 is en- One object of my invention is the provisionof 35 ergizcd m a Secondary Wihdihg 4 of transformmodulated carriercurrent transmitting and red s co e t o h a filt r c il 5 to ceivingapparatus which ha a, minimum nu b terminal B, with a filter condenserC4 connected of parts, and which can be quickly adjusted to acrossterminals B and function efficiently either as a receiver or as a The hat s of tubes V2 and V3 are e transmitter under widely different linecondigiZed from another s n a y gs of transtions. formerT l, the heatercircuits being omitted to A feature of my invention is the provision ofsimpl y the d novel means for controlling the unit by a push Terminal BSupp h v lta e to the screen button so as to enable the button to belocated grid of the tetrode V3. d t o h Coil 1 of a in the hand set butwithout requiring additional transmitting filter coupling C0i1T3 t0the'plate wires leading thereto. In accordance with this of tu t cat fwhi h'is connected to feature of my invention, a voltage divider isproground through a biasing resistor 8 and a byvided in the unit bywhich the potentials appass condenser plied to the electron tubes arecontrolled, which The resistor and the Winding o relay R is so arrangedthat the unit may be changed are connected in series across the supplytermifrom the receiving to the transmitting condition nals B and G andconstitute a voltage divider for by closing a circuit over a push buttoncontact providing a reduced voltage for energizing the which shunts aportion of the voltage divider. tubes VI and V2, relay R being normallyener- Another feature of my invention resides in gized, as shown, inparallel with tubes VI and V2. improved means for regulating the outputof The output or plate to cathode circuit of tube VI extends fromterminal B over resistors I and I I, front contact a of relay R to theplate of tube VI, the cathode of which is connected directly to ground.The input or grid to cathode circuit of tube VI includes resistor I2 andcondenser I3 and the winding Id of a receiving filter or coupling coilTI. The Winding I4 is tuned to resonance by a condenser C2, and iscoupled inductively to the line winding I5 which is tuned to resonanceby the condensers CI and connected over front contacts I) and d of relayR and condensers CI to the line terminals LI and L2.

The coupling coil Tl serves as a filter to selectively pass modulatedcarrier current, and tube VI is connected to function as a detector ofthe voice frequency components of the carrier current as will be readilyapparent. The carrier frequency component of the current in the outputcircuit for tube VI is by-passed through condensers I6 and I! to ground,while the voice frequency components pass through condenser l8 and again control resistor RR to ground, resistor RR having an adjustableconnection to the grid, or input circuit of tube V2.

The output circuit of tube V2 extends from terminal B over resistor ID,and the primary Winding of transformer T2 to the plate of tube V2, thecathode of which is connected through a biasing resistor I9 to ground,so that tube V2 amplifies the voice frequency currents and supplies themto the telephone receiver H connected to the secondary winding oftransformer T2,

The receiving system of Fig. 1 thus comprises the selective filter TI,the detector VI, audio amplifier V2, transformer T2 and the telephonereceiver H.

To establish the transmitting condition, the operator presses the buttonP on the hand set which shunts relay R by a low resistance circuitportion comprising primary Winding 20 of transformer T3 and themicrophone M of the hand set, through which sufficient current fiows torender the microphone active, but reducing the voltage across relay R tosuch a low value that tubes Vl and V2 are disabled, and relay Rreleases.

When relay R releases, its back contacts 0 and 6 connect the lineWinding 2I of the transmitting filter T3 through the condensers C3across the line terminals LI and L2, in place of the receiving filterTI, and the output circuit of tube V: is supplied with current fromterminal B over resistor 22, winding 23 of the receiving filter TI,resistor 24 and back contact a of relay R. The winding I4 of filter TIwhich is tuned by condenser C2 to the carrier current frequency and isconnected to the input circuit of tube VI, is thereby set intooscillation, and serves as a tank circuit to cause tube VI to supplycurrent to winding 23 of filter TI to maintain the oscillations. In thetransmitting condition, tube VI thus serves as an oscillator to generatecarrier current, but inasmuch as the circuit for winding I5 of filter TIis now open, the current in coil I l and condenser C3 will tend tooscillate at a slightly different frequency from that to which they areresonant in the receiving condition. It is desirable that the receivedand transmitted frequencies should be substantially the same, and tothis end compensation is provided by including the resistor 24 in thecircuit of winding 23 to shift the phase of the voltage fed back to thetank circuit of the oscillator by the required amount.

The carrier frequency voltage from the lefthand terminal of resistor 22to ground, due to the oscillations generated by tube VI, is impressedupon the gain control resistor RT through con densers 25 and 26, whichare of relatively small capacity. Resistor RT has an adjustableconnection for applying a portion of this voltage to the input circuitof tube V3, the output of which is supplied through the coupling coil T3and condensers C3 to the line terminals LI and L2. The secondary winding21 of transformer T3 is connected across resistors RT and 28, in series,and it will be seen that as the sound Waves to which microphone Mresponds cause current variations in winding 20, a voice frequencyvoltage is generated in winding 21 which varies the bias of the grid oftube V3 to correspondingly modulate the carrier current supplied by tubeV3 to the line circuit.

The resistor 28, by-passed by condenser 26, is included in the secondarycircuit of transformer T5 in order to permit the gain control resistorRT to be set at any position without altering the degree of modulationof the carrier frequency delivered by tube V3. To make this clear I haveshown a condenser CT in the drawing, representing the stray capacity ofthe grid of tube V3, which it will be seen shunts the upper portion ofresistor RT, and so tends to cause the change in the carrier voltageapplied to the input circuit of tube V3 in response to a given change inadjustment of resistor RT to be less than the change in voice frequencyvoltage, since the latter is not afiected materially by the p.esence ofthe small capacity CT. On the other hand, the presence of resistor 28 inthe circuit has but slight effect upon the carrier voltage due to thepro vision of the by-pass condenser 26. Resistor 28 reduces the effectof a change in adjustment of resistor RT with respect to the voicefrequency voltage, to correspond. with the reduced change in carriervoltage due to the effect of the stray capacity CT. That is to say, forany setting of resistor RT the ratio of the modulation voltage to thecarrier voltage is substantially constant.

The transmitting system of Fig. 1 thus comprises a source of carriercurrent comprising the oscillator tube VI, the tank circuit I4C2, andthe coupling coil 23; a source of voice fr;quency current comprising themicrophone M and transformer T6; a grid-modulated transmitting amplifierV3, and a transmitting filter T3.

It has been stated that the units are intended for use over existingcommunication circuits without interference with the services normallyusing such circuits. This is accomplished by the use of filters such asTI and T3 which are signed to present a high impedance to allfrequencies outside the band of frequencies em ployed by the units. Thetuned impedance presented to the line by the receiving filter TI tocurrents Within that band is also made relatively high in order topermit several units to be bridged across the line without causingserious attenuation of the transmitted signals.

Referring now to Fig. 2, this form of my invention differs principallyfrom Fig. 1 in that the change from the receiving to the transmittingcondition in response to the operation of the push-to-talk button P isobtained wholly by the resulting changes in the potentials applied tothe tubes without the use of a relay R as in Fig.1. I

In Fig. 2, the power supply unit comprising transformer T5 and tube V4,etc., is the same as in Fig. 1 except that its output terminals aredesignated B and N, the lower terminal N being made negative withrespect to ground by connecting resistors 3!, 32 and 33 across theterminals B and N and grounding the junction of resistors 32 and 33 asshown. Tubes Vi and V2 function as a detector and audio amplifier in thereceiving condition, and tube V3 as a grid modulated amplifier in thetransmitting condilion, as in Fig. 1, and since Fig. 2 lacks means forchanging the connections of tube Vi provided in Fig. 1 by relay R, aseparate oscillator tube V5 is provided having a tank circuit comprisingthe coil T5 and condenser 05, similar to the winding i l of filter Tiand condenser Cl, and the transmitting filter T3 is arranged to serve asa receiving filter as well, by the provision or a winding 34 tuned by acondenser C6 and loosely coupled to the line winding 2!.

The output circuit of tube V3 in Fig. 2 extends from terminal B throughwinding 1 to the plate of tube V3, and from its cathode through resistor8 to ground, and thence through resistor 33 to terminal N, and the gridof tube V3 is connected through resistors RT and 28 to terminal N, whichmaintains a negative biasing potential on the grid of tube V3 suflicientto cut off the plate current and render tube V3 inactive so long as theunit is in the receiving condition.

The output circuit for tube V5 extends from terminal B through resistor35 to the plate of tube V5, and from its cathode through the lowerportion of coil T5 to ground and thence through resistor 33 to terminalN. The grid of tube V5 is connected through resistor 36 to terminal N sothat tube V5 is also biased negatively to a point which renders itinactive when the unit is in the receiving condition.

In Fig. 2, the potential difference across resistor 33 is applied to thedetector tube Vi, the plate to cathode circuit of which extends fromterminal 13 over resistors 3i and 32 to ground, and thence in parallelwith resistor 33, from ground through the gain control resistor RR tothe plate of tube Vi, and from its cathode to terminal N. When modulatedcarrier current is received from the line by the filter T3, the resonantvoltage induced in its winding 34 is supplied through a small condenser30 to the grid of tube Vi, which is also connected through a highresistance 3? to terminal N, tube Vi serving as a detector, as inFig. 1. The carrier frequency components of the output of tube Vi areby-passed to terminal N through condensers 38 and 39, and the voicefrequency components are impressed on the gain control resistor RR andadjustably applied through condenser ifi to the input circuit of tubeV2. The output circuit of tube V2 in Fig. 2 extends from terminal Bthrough transformer T2 to the plate of tube V2 and from its cathode toground, and thence through resistor 33 to terminal N, and the voicefrequency components of the output of tube V2 are supplied throughtransformer T2 to the telephone receiver H.

In Fig. 2 the microphone M and winding 29 of transformed T6 areconnected across resistor 32 and are supplied with a low value ofcurrent in the receiving condition through resistors 31 and 33.

When button P is pressed to establish the transmitting condition, itscontacts connect terminals N and G together, short circuiting resistor33. This disables the detector tube Vi, the plate to cathode circuit ofwhich is connected across resistor 33, and the current through themicrophone M is increased to a suitable operating value, while thenegative bias on the grids of tubes V5 and V3 is decreased to a valueproper for their operation. The tank circuit comprising coil T5 andcondenser C5 is set into oscillation at the carrier frequency toenergize the input circuit for tube V5, the output of which is suppliedto the lower half of coil T5 to maintain the oscillations and isimpressed on the transmitter gain control resistor RT through condensers25 and 26. The voice frequency voltage developed by microphone M isimpressed by transformer T6 on resistors RT and 28 so that tube V3functions as a grid modulated transmitting amplifier as in Fig. 1, tosupply its output through the filter T3 to the line terminals Li and L2.

It will be seen that the filter T3 continues to supply voltage to thegrid of the detector tube VI, when button P is pressed, but the plate,grid and cathode of tube Vi are then grounded, and tube VI is therebydisabled and rendered incapable of energizing the input circuit of tubeV2. The plate circuit of tube V2 is constantly energized in Fig. 2, andit would be capable of energizing the telephone H if tube Vi were notfully suppressed or if voltage were supplied to the grid-cathode circuitof tube V2 by a stray field, and to prevent the response of the receiverH under such conditions, button P is provided with an additional contactin Fig. 2, by which transformer T2 is short circuited when button P ispressed.

In Fig. 2, the tuned impedance which the apparatus presents to the lineis made relatively high, as in Fig. 1, but since in Fig. 2 the samecoupling coil is used for transmitting, its impedance will generally behigher than the optimum value at which tube V3 delivers its maximumoutput, in which case the apparatus of Fig. 2 would not be capable oftransmitting as much power as that of Fig. 1. For this reason theamplifier tube V2 is arranged to be supplied with a higher plate voltagein Fig. 2 to increase the power supplied to the receiver H therebycompensating for the decreased power of the received signals.

Although I have herein shown only two forms of apparatus embodying myinvention, it is to be understood that various changes and modificationsmay be made therein within the scope of the appended claims Withoutdeparting from the spirit and scope of my invention.

Having thus described my invention, what I "claim is:

1. Carrier current communication apparatus comprising a telephonereceiver and a microphone, an electron tube detector, an electricalfilter including resonant circuit elements tuned to a carrier currentfrequency having input terminals for connection to a line circuit andoutput terminals connected to the input circuit of said detector tube,means for energizing said telephone receiver in response to the voicefrequency components of the output of said detector tube, a normallyinactive transmitter tube having an output circuit energized from adirect current source, said circuit including elements tuned to acarrier frequency and adapted to deliver the output of said transmittertube to said line circuit,

an input circuit for said transmitter tube connected to a normallyinactive source of carrier frequency current and also to a source ofvoice frequency current derived from said microphone, a Voltage dividerconnected across the terminals of said direct current source forsupplying a reduced voltage to said detector tube and said microphone,manually operable contact means for shunting a portion f said voltagedivider "to disable sa-id detector tube and to render the iniempheneeffective 't'o impress voice frequency variations"o'n'the input circuitof 'said't'r'ansmitt'e'r tube, and means responsive '-to the shunting'of said voltage divider by said push button circuit for rendering saidsource of'ca'rrie'r current efiective "to energze the input circuit ofsaid transmitter tube and to render said transmitter tube "effective todeliver its output to said line circuit.

2. Carrier current communication apparatus comprising a telephonereceiver and 'a 'microphone, an electron tube detector, an electricalfilter including resonant circuit elements tuned to a carrier currentfrequency having input terminals for connection to a line circuit andoutput terminals connected to the input circuit of saiddetector tube,means for energizingsaid telephone receiver in response to the voicefrequency components of the output of said detector tube, a normallyinactive transmitter tube having an output circuit energized from adirect current source, said circuit including elements tuned to acarrier frequency and adapted to deliver the output of said transmittertube to said line circuit, an input circuit for said transmitter tubeconnected to a normally inactive source of car-' rier frequency currentand also to a source of voice frequency current derived from saidmicrophone, a voltage divider connected across the terminals of saiddirect current source -forsupplying a reduced voltage to said detectortube, comprising a resistor and the Winding-of 'arelay in series,manually operable contact means for shunting the winding of said relayto effect its release and to disable said detector tube, and meanscontrolled by said relay when released for rendering said source ofcarrier current effective to energize the input circuit of saidtransmitter tube and to render said transmitter tube effective todeliver its output to said line circuit.

3. Carrier current communication apparatus comprising a telephonereceiver and a micro- -phone, an electron tube detector, an electricalfilter including resonant circuit elements tuned to a carrier currentfrequency having input terminals for connection to a line circuit andoutput terminals connected to the input circuit of said detector tubes,means for energizing said telephone receiver in response to the voicefrequency components of the output of said detector tube, a normallyinactive transmitter tube having an output circuit energized from adirect current source, said circuit including elements tuned to acarrier frequency and adapted to deliver the output of saidtransmittertube to said line circuit, an input circuit -forsaidtransmitter tube connected to a normally inactive source of carrierfrequency current and also to a source of voice frequency currentderived from said microphone, a voltage divider connected across theterminals of said direct currentsource for supplying a reduced voltageto said detector tube, means responsive to the operation of said pushbutton for shunting a portion of said voltage divider to disable saiddetector tube, and means including said voltage divider when shunted forrendering said-source of carrier current and said microphone efiectiveto energize-the input circuit of said transmitter tube andto render saidtransmitter tube eiiective tosupplymodulated carrier current to saidline circuit.

4. In combination with a telephone handset "comprising a receiver, amicrophone, and apush button; a detector tube having an input circuitcoupled to a line circuit and an output circuit 'eouple'd to saidtelephone receiver, a normally inactive source of carrier current, anormally in- -'active 'transmi'tter'tube for supplying modulated saidvoltage divider across which said detector tube is connected to disablesaid tube, and means respons'ive'to the shunting of said portion forrendering said carrier current source efiective to energize the inputcircuit of said transmitter tube and for causing said transmitter tubeto supply modulated carrier current to said line circuit.

5. Carrier current communication apparatus comprising a telephonereceiver and a microphone, a detector tube, a transmitter tube, a sourceof direct current for energizing said tubes, a relay normally energizedfrom said source in series with a resistor, an input circuit for thedetector tube including resonant circuit elements tuned to a carrierfrequency and coupled to a line circuit over front contacts of saidrelay, a

first output circuit for said detector tube connected across the windingof said relay over a front contact thereof and arranged to supply voicefrequency components of its output to said telephone receiver, a secondoutput circuit for said tube connected to said source over back contactsof said relay and arranged to supply carrier frequency output of saidtube to its input circuit to cause oscillations of a frequencydetermined by said resonant circuit elements, means for sup plyingoscillations from said second output circuit to the input circuit ofsaid transmitter tube and for modulating said oscillations in accordancewith current supplied by said microphone, an output circuit for saidtransmitter tube connected'to a tuned circuit connected across said linecircuit over back contacts of said relay, and means including a normallyopen manually operable contact in series with said microphone forshunting said relay to effect its release and to render said microphoneeffective to modulate the oscillations supplied to said transmittertube.

6. Carrier current communication apparatus "comprising a telephonereceiver, a microphone and a push button, a relay controlled by saidpush button, a detector tube having an input circuit coupled to a linecircuit over front contacts of said relay and an output circuit coupledto said telephonereceiver over a front contact of said relayya source ofcarrier current controlled 'over a back contact of'said relay, atransmitter tube having an input circuit coupled to said source ofcarrier current and to said microphone, and an output-circuit coupled tosaid line circuit over back contacts of said relay, a source of directcurrent for energizing said tubes across which the winding of said relayis connected in series with a'resistor, a circuit controlled by saidpush button including said microphone for shunting said relay to effectits release and at the same time energizing the microphone to render itef- -fective to modulate the carrier current supplied to "-saidtransmitter tube.

[Carrier current communication apparatus comprising a telephone receiverand a microphone, a-detector tube having an input circuit coupled to aline circuit andan output circuit coupled to said telephone receiver, atransmitter tube having an output circuit coupled to said line circuit,an oscillator tube for supplying carrier frequency current to the inputcircuit of said transmitter tube, means controlled by said microphonefor modulating the current supplied to the input circuit of saidtransmitter tube, a source of direct current for energ z ng said tubesand said microphone, a voltage divider connected across the terminals ofsaid source which normally provides a reduced voltage for energizingsaid detector tube and applies a biasing voltage to the input circuit ofsaid transmitter tube sufiicient to render such tube inactive when thedetector tube is energized, and manua ly operable means for shunting aportion of said voltage divider to disable said detector tube and toremove said bias to render said transmitter tube effective to supplymodulated carrier current to said line circuit.

8. Carrier current communication apparatus comprising a telephonereceiver, a microphone, a detector tube having an input circuit coupledto a line circuit and an output circuit coupled to said telephonereceiver, a normally inactive transmitter tube having an output circuitfor supplying modulated carrier current to said line circuit, means forsupplying carrier current to the input circuit of said transmitter tubeand for modulating said current by said microphone, a source of directcurrent for energizing said tubes and said microphone, a voltage dividerconnected across the termina s of said source which normally provides areduced voltage for energizing said detector tube, means including saidvoltage divider for preventing said transmitter tube from supplyingcurrent to said line circuit when said detector tube is energized, andmanually operable means for shunting a portion of said voltage dividerto disable said detector tube and at the same time render thetransmitter tube effective to supply current to said line circuit.

9. Carrier current communication apparatus comprising a telephonereceiver and a microphone, a detector tube having an input circuitcoupled to a line circuit and an output circuit coupled to saidtelephone receiver, a normally inactive transmitter tube having anoutput circuit for delivering modulated carrier current to said linecircuit, a gain control resistor associated With the input circuit ofsaid transmitter tube for regulating the current delivered by said tube,a source of carrier current, means for supplying carrier current to saidgain control resistor from said source and for also supplying voicefrequency current thereto derived from said microphone to modulate saidcarrier current, manually operable contact means for disabling saiddetector tube and at the same time rendering said transmitter tubeactive, and means including a fixed resistor in the circuit by Whichvoice frequency current is supplied to said gain control resistor toreduce the effect of a change in adjustment thereof to compensate forthe reduced effect of said change in adjustment upon the carrier currentsupplied to the input circuit of said transmitter tube due to theshunting of said gain control resistor by stray capacity.

10. Carrier current communication apparatus comprising a telephonereceiver and a microphone, a detector tube having an input circuitcoupled to a line circuit and an output circuit coupled to saidtelephone receiver, a normally inactive transmitter tube having anoutput circuit for delivering modulated carrier current to said linecircuit, a gain control resistor having an adjustable connection to theinput circuit of said transmitter tube for regulating the currentdelivered by said tube, a source of carrier current having a connectionto the input circuit of said transmitter tube through stray capacity,means for supp ying carrier current to said gain control resistor fromsaid source and for also supplying voice frequency current theretoderived from said microphone to modulate said carrier current, meansincluding a fixed resistor in the circuit by which voice frequencycurrent is supplied to said gain control resistor to compensate for saidstray capacity, andmanually operable means for rendering saidtransmitter tube active to deliver its output current to said linecircuit and at the same time disabling said detector tube to render saidtelephone receiver nonresponsive thereto.

11. Carrier current communication apparatus comprising a telephonereceiver and a microphone, a detector tube having an input circuitcoupled to a line circuit and an output circuit coupled to saidtelephone receiver, a normally inactive transmitter tube having anoutput circuit for delivering modulated carrier current to said linecircuit, means for supplying carrier current to the input circuit ofsaid transmitter tube and for also supplying voice frequency currentthereto derived from said microphone to modulate said carrier current,and manually operable means for shunting said detector tube by arelatively low resistance circuit portion including said microphone torender the microphone effective to supply voice frequency current to theinput circuit of said transmitter tube and to render said tube effectiveto deliver its output current to said line circuit and at the same timedisabling said detector tube to render said telephone receivernonresponsive thereto.

12. In combination with a telephone hand set comprising a receiver and amicrophone, a source of direct current, a voltage divider connectedacross the terminals of said source, a detector tube having an inputcircuit coupled to a line circuit and a plate to cathode circuitconnected between a point on said voltage divider and the negativeterminal of said source and coupled to said telephone receiver, anoscillator tube for generating carrier current and a transmitter tubefor delivering said current to said line circuit, each of which has itsplate connected to the positive terminal and its grid connected to thenegative terminal of said source with its cathode connected to a pointon said voltage divider to bias the tube to cut off and render itinactive, a gain control resistor for regulating the input circuit forsaid transmitter tube which is supplied with carrier current by saidoscillator tube when active and by voice frequency current by themicrophone for modulating said carrier current, and manually operablemeans for connecting a point on said voltage divider to said negativeterminal to decrease the bias on the oscillator and transmitter tubes torender said tubes active and to short circuit said detector tube toprevent the response of said receiver to the current supplied to theline circuit by said transmitter tube.

13. In combination with a telephone hand set comprising a receiver and amicrophone, a source of direct current, a voltage divider connectedacross the terminals of said source, a detector tube having an inputcircuit coupled to a line transmitter tube for delivering said currentto said line circuit, each of which has its plate connected to thepositive terminal-and its grid connected to the negative terminal'ofsaid source, with its cathode connected to a point on said voltagedivider tobias the tube to cut off and render it inactive, means forsupplying carrier current from the output of said oscillator tube whenactive to the in-- i put circuit of sa1d transmitter tube, and for mod--ulati-ng said current by voice frequency current derived from saidmicrophone, manually operable means for connecting a point on saidvoltage di- 2b" 12 vide'r' t'o said ne a'tive on saidoscillator and saidtubes active, and means-for short circuiting said receiver to preventitsrespo'nse "When current is-being' supplied to the line-circuit bysaidtran'smitter tube.

K. CROOKS.

REFERENCES CITED The f'foll owing references are of record in the fileof this patent: V

UNIT-ED STATES PATEN -I S Numbe V Name lfiate' V A 2529!);487" Moorescram-"ce se oer. 2'0, r942 .IFQREIGN EATENTS Number country have105,358 Australia e -i May- 24, 11937 terminal tode'crea's'e thebias'transmitter tubes "to render

